Metal nanoparticles, particles of elemental metal in pure or alloyed form with a dimension less than 100 nm, have unique physical, chemical, electrical, magnetic, optical, and other properties in comparison to their corresponding bulk metals. As such they are in use or under development in fields such as chemistry, medicine, energy, and advanced electronics, among others.
Synthetic methods for preparing metallic nanoparticles are typically characterized as being “top-down” or “bottom-up” and comprise a variety of chemical, physical, and even biological approaches. Top-down techniques involve the physical breakdown of macroscale or bulk metals into nanoscale particles, using a variety of energy inputs. Bottom-up methods involve the formation of nanoparticles from isolated atoms, molecules, or clusters.
Physical force methods for top-down metal nanoparticle synthesis have included milling of macroscale metal particles, laser ablation of macroscale metals, and spark erosion of macroscale metals. Chemical approaches to bottom-up synthesis commonly involve the reduction of metal salt to zero-valent metal coupled with growth around nucleation seed particles or self-nucleation and growth into metal nanoparticles.
While each of these methods can be effective in certain circumstances, each also has disadvantages or situational inapplicability. Direct milling methods can be limited in the size of particles obtainable (production of particles smaller than ˜20 nm is often difficult) and can lead to loss of control of the stoichiometric ratios of alloys. Other physical methods can be expensive or otherwise unamenable to industrial scale. On the other hand, bottom-up chemical reduction techniques can fail in situations where metallic cations are resistant to chemical reduction. Mn(II) for example is virtually impervious to chemical reduction, making this approach inapplicable to the preparation of Mn0 nanoparticles or nanoparticles of Mn0-containing alloys.
Manganese-bismuth (MnBi) is one example, among many, of a zero-valent metal alloy whose nanoparticulate form is of considerable interest in applied technological fields. MnBi has been shown to have very high coercivity, a ferromagnetic property in which a magnet is strongly resistant to de-magnetization by an opposing magnetic field. Current state-of-the-art, high-coercivity magnets, or “hard magnets”, which are required in a variety of advanced electronic applications, typically include expensive rare-earth metals such as in the neodymium iron borate magnet. MnBi nanoparticles of consistently small dimension are predicted to have coercivities that rival or exceed those of materials such as neodymium iron borate.